Rotary encoders have many applications, including detection of vehicle steering wheel angular rotation. Typical prior art rotary encoders include a first encoder wheel attached to a rotating shaft. The periphery of the first encoder wheel comprises alternating equally-spaced N-pole and S-pole magnetic regions. A first magnetic detection sensor, such as a magnetoresistor or Hall sensor, is positioned adjacent the periphery of the first encoder wheel and produces a signal responsive to the passage of magnetic regions adjacent the sensor as the shaft is rotated. Each sensed magnetic region corresponds to rotation of the shaft by an incremental number of degrees equal to 360.degree. divided by the total number of magnetic regions.
Although this information enables detection of angular rotation of the shaft, it does not enable detection of a neutral position corresponding to a predetermined angular position. Therefore, a second encoder wheel is required. It is attached to a shaft and comprises a single magnetic region located within a defined portion along the encoder wheel periphery.
The angular position of the shaft which aligns the single magnetic region of the second encoder wheel adjacent a second magnetic detection sensor is referred to as the neutral position of the shaft and corresponds to a predetermined angular position. The neutral position of a steering wheel corresponds to the angular position of the steering wheel resulting in a straight direction of vehicle travel. Once the neutral position is identified, absolute angular positions of the steering wheel can be continuously identified in accordance with the direction of shaft rotation and the number of magnetic regions detected by the first sensor.